Having the train 'under full control' refers to the train moving at a slow speed that allows the operator to manage and respond quickly to any required stops or changes.
When the rules state that "train operators must approach all STOP signals, train ahead, junctions, and terminals with the train under full control," it most nearly means that the train is moving at a speed where the operator can effectively manage and stop it if necessary. This implies that the train cannot be moving at its maximum speed, as it would not allow for the quick responses required in the situations mentioned.
Therefore, the correct interpretation of having the train "under full control" aligns most closely with the train moving at a slow speed, allowing the operator to take action promptly in case of an unexpected event.